Throughout history, breakthroughs in the field of microscopy often brought about groundbreaking advancement in scientific research. For example, with the development of scanning tunneling microscopy (STM), electron microscopes (EM) and related techniques, nanotechnology has become an emerging technology with enormous potential to alter our way of life in decades ahead. More than 400 years ago, the first optical microscope was invented to visualize objects invisible to the naked eyes. Until now, the optical microscope remains one of the most widely used microscopes in both industry and academia among three main branches of microscopes: optical microscope, EM (both the scanning electron microscope, SEM and the transmission electron microscope, TEM) and the family of scanning probe microscope (SPM). Optical microscopes (OM) offer the merits of fast speed, large-area imaging and ease of sample preparation; however, the diffraction barrier prevents conventional optical microscopes from distinguishing the nanostructures below 200 nm using visible light.
Near-field scanning optical microscopy (NSOM) has circumvented the diffraction limit by bringing the tips to the proximity of sample surface of interest in the region within ˜10 nm of the tip or nano-antenna, however it is restricted to a small scanning area. Recently, the diffraction limit has been successfully surpassed by super-resolution fluorescence microscopy including stimulated emission depletion microscopy (STED) or reversible saturable optical fluorescence transitions (RESOLFT), photo-activation localization microscopy (PALM) or fluorescence PALM (FPALM), saturated structured illumination microscopy (SSIM) and stochastic optical reconstruction microscopy (STORM). Although the resolution as high as 20 nm has been achieved by the aforementioned methodologies in life science researches, the requirement of fluorophores precludes its applications in the samples which cannot be labeled with fluorophores, such as silicon based semiconductors. On the other hand, EM employs short-wavelength electron beams to illuminate samples and therefore achieves the resolution better than 50 pm for TEM. Nevertheless, it is expensive to build and maintain. SPM such as STM and atomic force microscope (AFM) acquires topographical image and other images by utilizing a physical probe to touch and feel specimen surfaces directly. As such, images containing a variety of surface properties such as topographical, chemical, electrostatic, magnetic and thermal properties, etc., can be presented directly with sub-molecular resolution.
However, the key problems with SPM are the slow scanning speed, small scan size and small depth of field. For example, a commercial AFM takes several minutes to obtain a high-quality image with the maximal scan size less than 80 by 80 μm2. Although cantilevers with high resonant frequency are being developed to increase the AFM scanning speed since more than 15 years ago, the serial nature of single tip based design restricted the scanning area within 40 by 40 μm2 in one pass until now. Parallel imaging with multiple tips has been enabled by utilizing an integrated piezo-resistive sensor and integrated ZnO actuator, whereby the noise limit the resolution. Further, the complex design and setup make it difficult to be accessed by most researchers. Thus, up to date, a simple imaging technology which enables topographical imaging with sub-diffraction resolution over large areas (up to millimeter scale) remains challenging.
Recently, the development of polymer pen lithography (PPL) has increased the throughput of traditional single tip based dip pen nanolithography by over three orders of magnitude by utilizing a tip array comprised of hundreds of polydimethylsiloxane (PDMS) tips to ‘write’ ink on substrate surfaces simultaneously.
For AFM, the specimen surface information is reflected by the deflection or torsion of a flexible cantilever, which are detected by monitoring the tiny movement of a laser beam projected on the back side of the cantilever. Nevertheless, multiple laser beams for multiple tips parallel imaging are difficult to implement since multiple lasers may interfere with each other in a small confined space.
Since microscopy provides smart “eyes” for researchers to visualize and explore micro/nanomaterials, developing a facile microscopy which enables large-area imaging without the restriction of diffraction limit is a longstanding goal in nanotechnology community. Although the advancement of microscopy has revolutionized the scientific research in many fields in the past, as described above, state-of-the-art microscopy imposes tradeoffs between throughput, resolution and sample surface information acquired.